Radar for the detection of fixed targets in clutter

ABSTRACT

A high range resolution FM-CW radar system, having a range resolution cell on the order of the minimum distance between two highly reflective points of a target. Circuits are included for evaluating the difference between the echo intensity received from a resolution cell containing a highly reflective point of a target and that received from a resolution cell located at the same distance but containing only clutter.

BACKGROUND OF THE INVENTION

This invention concerns radar systems generally and more specificallyrelates to enhanced detection of fixed targets in the presence ofundesired echoes (clutter).

The term clutter refers to unwanted echoes such as produced by theground, grass and foliage, and sea surface as well as atmosphericprecipitation. Useful or desired echoes coming from targets of interestare often masked by clutter echoes, hence the need to separate theuseful echoes from the clutter echoes. In the case of moving targets,use can be made of the Doppler effect in a number of known ways todetect them within the clutter. However, this solution is obviously notapplicable to fixed targets. The manner in which the invention advancesthe art by providing novel means for detection of fixed targets inclutter will be described hereinafter.

SUMMARY OF THE INVENTION

The general objective of the invention may be said to have been theprovision of a radar allowing fixed targets to be detected in the midstof clutter.

According to one characteristic of the invention, the radar used is afrequency-modulated, continuous-wave radar having high range resolution,on the order of the minimum distance between bright points of targetshaving large radar cross section. The discrimination between targets andclutter is based on the echo intensities received from a resolution cellcontaining a bright point of a target and a resolution cell located atthe same range but containing only clutter.

Other characteristics and advantages of this invention will be broughtout in the following description made in connection with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a typical frequency deviation curve of the signaltransmitted by a frequency-modulated radar.

FIG. 2 is a graph indicating, for various clutter conditions, and as afunction of range, the minimum target radar cross-section detectable ina resolution cell by the radar according to the invention.

FIG. 3 shows the basic schematic block diagram of the radar according tothe invention.

FIGS. 4(a) through 4(d) show waveforms of signals generated at selectedpoints within the radar system according to the invention.

FIG. 5 shows a typical frequency spectrum corresponding to the analysisof a portion of the range covered by the radar according to theinvention.

DETAILED DESCRIPTION

The principle employed for detection of fixed targets (as hereinbeforesummarized) in the midst of clutter is based on the observed differencesbetween the RF diffusion characteristics of the desired targets andthose of the clutter. Targets of interest (tanks, vehicles, buildings, .. . ) have edges and approximately flat surfaces defining, from thepoint of view of echoes, a small number of diffusers, each having arelatively large radar cross-section and separated in range on the orderof one to several meters. Such a set of diffusers determines practicallythe total radar cross-section of the desired target.

Clutter echoes, on the contrary, generally are composed of a largenumber of diffusers located relatively close to each other, but eachhaving a small radar cross-section.

According to the invention, fixed targets in the midst of clutter aredetected by the radar of high range resolution, on the order ofmagnitude of the minimum distance between bright points of the target(diffusers having a large radar cross-section), for example on the orderof one meter. Thus, the intensity of the echo received from a resolutioncell containing a bright point of the target will be greater than thatof an echo received from a resolution cell located at the same distancebut containing only clutter.

The high-resolution radar used in the invention is of thefrequency-modulated continuous-wave (FM-CW) type. The followingmathematical expressions are used to determine the radar rangeresolution as a function of the carrier wave modulation characteristics.

We shall call:

φ the phase shift between the echo received from a point and thetransmitted wave;

D the distance between the radar and the said point of reflection;

F the frequency of the transmitted wave;

c the propagation velocity; and

t the time.

The phase shift φ is equal to:

    φ=4πFD/c                                            (1)

This phase shift varies as a function of time if F or D varies andresults in an apparent frequency f of the echo:

    f=(1/2π)(dφ/dt).

This expression may be rewritten, from equation (1) as follows: ##EQU1##

Limiting the discussion to fixed targets for the moment, the apparentfrequency f is a function only of the transmitted wave frequency F, asfollows:

    f=(2D/c)(dF/dt).

We shall assume that the transmitter is frequency-modulated linearlywith according to a triangular waveshape a deviation ΔF for a time T, asshown in FIG. 1. The frequency F of the transmitted wave varies linearlybetween values F_(o) and F_(o) +ΔF during the time period T. We can thenwrite:

    dF/dt=ΔF/T.

Whence:

    f=(2D)(ΔF)/cT                                        (2)

And thus:

    D=cTf/2ΔF.

Range resolution, which will be called ΔD, is a function of theuncertainty of measurement of the apparent frequency f. Letting Δf bethis uncertainty over the frequency f, we can write:

    ΔD=(cT)(Δf)/2ΔF.

The time interval available for the measurement of f being equal to T,the uncertainty of measurement Δf will therefore be on the order of 1/Tif the measurement is made on a single frequency deviation. The rangeresolution ΔD is therefore given by:

    ΔD=c/2ΔF.                                      (3)

For example, a range resolution ΔD of 0.5 m is obtained with a frequencydeviation ΔF of 300 MHz.

The following mathematical expressions are used to determine the ratioα=useful signal/clutter signal.

We have:

    α=σ/σ.sub.c,

in which

σ_(c) represents the radar cross-section of the clutter in a resolutioncell of size ΔD located at a distance D from the radar, and

σ represents the radar cross-section of the target in the saidresolution cell.

σ_(c) may be then written as

    σ.sub.c =σ.sub.o (d)(θ)(ΔD)

where

σ_(o) is the clutter density per unit area, and

θ is the bearing beamwidth of the antenna lobe.

By replacing ΔD by expression (3), we have:

    σ.sub.c =(σ.sub.o)(D)(θ)(c)/(2ΔF).

Thus, α can be written in the form ##EQU2##

As an example, the numerical values indicated below, when substitutedinto equation (4), give a useful signal/clutter signal ratio of 22.86,or 13.6 dB. These substitution example values are

θ=1° (0.0175 rad),

ΔF=300 MHz,

σ_(o) =0.005 m² /m², (average vegetation)

σ=1 m², and

D=1 km.

Expression (4) can be used to calculate the minimum radar cross-sectionσ_(min). of a target detectable in a resolution cell, as a function ofthe range for various conditions of clutter, as follows:

    σ.sub.min. =(α)(σ.sub.o)(θ)(c)(D)/(2ΔF).

Calculations have been made using the following numerical values:

α=20

θ=1°

ΔF=300 MHz.

The results are shown on the diagram of FIG. 2 for three different typesof clutter:

    ______________________________________                                        Tilled ground:    σ.sub.o = 0.001 m.sup.2 /m.sup.2 (-30 dB);            Average vegetation:                                                                             σ.sub.o = 0.005 m.sup.2 /m.sup.2 (-23 dB);            Wooded (Forested) ground:                                                                       σ.sub.o = 0.032 m.sup.2 /m.sup.2 (-15                 ______________________________________                                                          dB).                                                    

Although moving targets can be detected, as is well known, by a Dopplerradar, they will also be detected by the high-resolution radar accordingto the invention. The range error introduced by the Doppler effect isnegligible, as will be explained below.

Consider:

    F.sub.D =2v/λ

in which,

F_(D) is the Doppler frequency of the target;

v is the radial velocity of the target; and

λ is the wavelength.

By making λ equal to three centimeters (10 GHz), we obtain a Dopplerfrequency of 66 Hz for a radial velocity of 1 m/s. From the expression(2), we can calculate the range error introduced by that Doppler effect.Using ΔF equal to 300 MHz and T equal to 1 ms, we find an error of only33 millimeters for a velocity of 1 m/s.

FIG. 3 shows the basic block diagram of the high-resolutionfrequency-modulated, continuous-wave radar in accordance with theprinciples of the invention, permitting the detection of fixed targetsin the midst of clutter. It includes a pilot oscillator 1,frequency-modulated linearly under the control of a clock 2. The curvegiving the value of the frequency F of oscillator 1 as a function of thetime t has the shape of a triangular wave, as shown in FIG. 4(a). Thefrequency F varies from F_(o) +ΔF/2 to F_(o) -ΔF/2 over a period T.Oscillator 1 is followed by a microwave transmitter 3, for example oneincluding a travelling-wave tube. Transmitter 3 is connected to arotating antenna through a duplexer 5. The echoes received by theanntenna 4 are delivered at output 17 of duplexer 5, and are then mixedwith a fraction of the transmitted signal in a mixer 6. This fraction ofthe transmitted signal is tapped off by means of a directional coupler7. The output of mixer 6 is connected to the input of a video amplifier8 delivering a signal having a frequency equal to the difference betweenthe transmitted frequency and the received frequency. This differencefrequency is referred to as the apparent frequency f of the echo. Thebandwidth of amplifier 8 is equal to the range of expected variation ofthe said apparent frequency, which will be called (f_(max). -f_(min).).The apparent frequency f_(max). corresponds to the maximum detectionrange, which will be called D_(max). The apparent frequency f_(min).corresponds to the minimum detection range, which will be calledD_(min).. The voltage gain of amplifier 8 is proportional to a power ofthe frequency, so as to provide automatic gain control as a function ofthe range.

The frequency f signal delivered by amplifier 8 is mixed, by means of asingle-sideband mixer 9, with the output signal of a frequencysynthesizer 10 whose frequency can vary from F_(o) +f_(min). to F_(o)+f_(max). in steps of Δf_(o), under the control of clock 2. FIG. 4(c)shows the output frequency of synthesizer 10 as a function of time.Jumps in frequency occur when the sign of the slope of the sawtoothcurve shown in FIG. 4(a) changes, and are therefore spaced by a timeperiod T. Mixer 9 is designed to deliver a signal with a bandwidthΔf_(o). The output signal from the said mixer 9 is received by aspectrum analyzer 11, controlled by clock 2. Each step Δf_(o) defines aportion of distance ΔD_(o) within the total detection zone going fromD_(min). to D_(max). Thus, on each frequency step of synthesizer 10, thespectrum analyzer 11 analyzes the corresponding portion of distanceΔD_(o). The total detection zone is analyzed by successive portions ofdistance ΔD_(o) during the frequency sweep of the synthesizer 10, thisfunction being basically handled by clock circuit 2.

The spectrum analyzer 11 will preferably be of the digital type.Processing is delayed by a time T, this being accomplished by theexpedient (known per se) of using two alternately operating memorieswithin 11. A time T is thus available to the spectrum analyzer todeliver the frequency spectrum of a signal of bandwidth Δf_(o) with aresolution equal to the frequency resolution Δf of the radar, thisfrequency resolution Δf corresponding to the range resolution ΔD. Thespectrum analyzer should therefore deliver, in a time T, N lines (whereN=Δf_(o) /Δf) corresponding to N distances each separated by ΔD within aportion of distance ΔD_(o). FIG. 5 shows an example of a frequencyspectrum for a given portion of the range. It can be shown that lowamplitude lines correspond to clutter, and larger amplitude linesindicate a desired target, the discrimination occuring above a thresholdof amplitude S.

A threshold circuit 12 (cutting off below level S), placed at the outputof the spectrum analyzer 11, permits the clutter signal to beeliminated, and delivers a brightness signal to a cathode ray display 13when the amplitude of the signal delivered by the spectrum analyzer isabove the said threshold S. The electron beam of the display CRT 13 isdeflected in bearing by a sweep circuit 14 controlled by an angle datatake-off circuit 15 which determines the position of the antenna 4, andin range by a range sweep circuit 16 controlled by the clock 2. Blankingsignals are also sent from the clock 2 to the display 13 during the beanreturn operation, thus the clock 2 performs as a master synchronizingcircuit. Other blanking signals, shown in FIG. 4(b), are sent from clock2 to amplifier 8 at the time of the frequency sweep reversal ofoscillator 1 so as to exclude noise signals present at that moment.

The radar according to the invention can operate in two different modes:

(1) Surveillance of the total detection zone; and

(2) Fine analysis of a selected portion of distance ΔD_(o).

Both of these operating modes are explained below:

Surveillance of the Total Detection Zone

In the surveillance mode, antenna 4 rotates and display 13 displays theentire zone under surveillance, in bearing and in range, in the PPImode. When a target is detected in a given portion of distance ΔD_(o), asignal point is illuminated on display 13 in the direction given by theposition of the antenna 4, and at a range corresponding to the analyzedportion of distance ΔD_(o). To this end, the synthesizer 10 deliversfrequencies changed by steps, as explained above and as shown in FIG.4(c). The range sweep circuit 16 delivers step voltages, as shown inFIG. 4(d). The frequency steps of the synthesizer 10 and the voltagesteps of sweep circuit 16 are synchronized. Any spectral line receivedby the threshold circuit 12 exceeding the threshold S will illuminatethe display. All spectral lines belonging to the same portion ofdistance ΔD_(o) will illuminate the screen of the display at a singlerange determined by the voltage step which is supplied by the sweepcircuit 16, resulting in a single bright spot on the screen. The rangeresolution is then ΔD.sub. o.

Fine Analysis of a Selected Portion of Distance ΔD_(o)

Fine analysis of a portion of distance ΔD_(o) can indicate the numberand type of targets contained in this portion. This mode of operationrequires modification of some clock signals. The antenna rotation issearchlighted (brought to a stop in the chosen direction) and theselected portion of distance ΔD_(o) is spread over the entire screen.The frequency delivered by the synthesizer 10 is fixed and correspondsto the portion of range in question. Therefore the spectrum analyzercontinuously analyzes this same portion of range. The range sweepcircuit 16 supplies a sawtooth voltage corresponding to a sweep of theentire screen. Thus, each line delivered by the spectrum analyzer 11that exceeds the threshold S will illuminate a point on the screen, atthe corresponding range. The range-resolution is then equal to ΔD.

Although this invention has been described in connection with aparticular embodiment, it is clearly not limited to the said embodimentand is capable of variations and modifications within its inventivescope. For example, it will be evident that the conventional CRT sweepcan be replaced by a television-type (raster) sweep, with the use of abuffer storage element.

What is claimed is:
 1. A high resolution radar for the detection offixed targets in clutter, comprising:first means associated with saidradar comprising an angularly scannable antenna and means for generatingand radiating a continuous-wave frequency-modulated transmitted signal,and means for receiving echo signals from targets and clutterilluminated by said transmitted signals; second means associated withsaid means for receiving echo signals and for successively selectingresolution cells in range containing echo signals corresponding to bothclutter and fixed targets; third means responsive to said echo signalsfrom said second means for successively producing a plurality ofspectral line signals each of amplitude, each representing the magnitudeof a corresponding frequency component within a given resolution cell,said spectral lines having greater amplitudes as they result from fixedtargets than as resulting from clutter echoes; and fourth meansresponding substantially only to said spectral lines of greateramplitude for producing an output identifying a fixed target mixed withclutter within a corresponding resolution cell.
 2. Apparatus accordingto claim 1 in which said frequency modulation of said transmitted signalis effected in accordance with a linearly varying frequency-versus-timefunction.
 3. Apparatus according to claim 2 in which said frequencymodulation frequency-versus-time function is a triangular wave having anenvelope with substantially linear increasing and decreasing slopes. 4.Apparatus according to claim 1 further comprising cathode ray displaymeans having range and angle scanning means synchronized with saidenvelope of said frequency-vesus-time function and the instantaneousangle of scan of said scannable antenna, respectively, said displaymeans being responsive to the output of said fourth means to produce anintensified point on the screen of said display.
 5. A radar systemaccording to claim 4 in which said second means comprises means forsuccessively analyzing at least a portion of the total useful range ofsaid system D_(o) by successive increments of ΔD_(o) ;in which saidsecond means further includes a frequency synthesizer whose frequencyvaries by steps of Δf_(o) corresponding to the variation of the apparentfrequency of target echoes over said ΔD_(o), said steps being spaced bytime T and beginning at the time of each change of frequency slope ofsaid frequency-versus-time modulation function; in which asingle-sideband mixer is included connected to receive the output ofsaid frequency synthesizer and the received apparent target echofrequency f to produce a signal of bandwidth Δf_(o) ;and in which saidthird means is a spectrum analyzer responsive to said single-sidebandmixer to provide, in said period T, said spectral line signals of saidΔf_(o), said spectral lines comprising N lines separated in frequency byΔf, which value Δf is a function of the effective range resolution ofthe system ΔD.